The magnitude of the exercise pressor reflex is influenced by the active skeletal muscle mass in the decerebrate rat

Abstract

The exercise pressor reflex is composed of two components, namely the muscle mechanoreflex and the muscle metaboreflex. The afferents evoking the two components are either thinly myelinated (group III) or unmyelinated (group IV); in combination they are termed “thin fiber afferents.” The exercise pressor reflex is often studied in unanesthetized, decerebrate rats. However, the relationship between the magnitude of this reflex and the number of thin fiber afferents stimulated by muscle contraction is unknown. This lack of knowledge prompted us to test the hypothesis that the magnitude of the exercise pressor reflex was directly proportional to the amount of muscle mass activated. Muscle mechanoreceptors were stimulated by stretching the calcaneal tendon. Likewise, muscle metaboreceptors were stimulated by injecting lactic acid into the arterial supply of the hindlimb muscles. In addition, both muscle mechanoreceptors and metaboreceptors were stimulated by statically contracting the hindlimb muscles. We found that simultaneous bilateral (both hindlimbs) stimulation of thin fiber afferents with stretch, lactic acid, and static contraction evoked significantly greater pressor responses than did unilateral (one hindlimb) stimulation of these afferents. In addition, the magnitude of the pressor responses to bilateral simultaneous stimulation of thin fiber afferents evoked by stretch, lactic acid, and contraction was not significantly different from the magnitude of the sum of the pressor responses evoked by unilateral stimulation of these afferents by stretch, lactic acid, and contraction. We conclude that the magnitude of the exercise pressor reflex and its two components is dependent on the number of afferents stimulated.

INTRODUCTION

The exercise pressor reflex is widely believed to play an important role in evoking cardiovascular responses to exercise. The reflex, which arises from the contraction-induced stimulation of group III and IV muscle afferents, has been shown to increase arterial blood flow to the working muscles in both healthy humans and animals (2, 18). One issue that has arisen from the study of the exercise pressor reflex is whether the amount of muscle mass contracted influences its magnitude. The findings have been controversial, and, for the most part, the studies have been conducted on humans. For example, Lind and McNicol (14) have argued that total tension development, regardless of active skeletal muscle mass, determined the magnitude of the pressor response to static exercise. This finding, however, was in conscious humans in whom central command may have played a role in evoking the responses to exercise (5, 8). In order to study the exercise pressor reflex, one must isolate its effects from those caused by central command, which is defined as the parallel activation of central motor drive and autonomic neural circuits that evoke cardiovascular responses to exercise (8). This isolation is best achieved by using anesthetized or decerebrated animal preparations (3, 11, 15, 16, 22).

Using anesthetized animal preparations, McCloskey and Streatfeild (15) compared the pressor response between unilateral and bilateral contraction in dogs and cats. The authors found that the pressor response to bilateral contractions was larger than the pressor response to unilateral contraction. Although these findings provided insight into the effect of muscle mass on the pressor response to contraction, these data were qualitative and were not analyzed statistically. In addition, developed tension was neither controlled nor reported, raising the possibility that any difference was due to differences in mechanical/metabolic stimuli. Finally, the experiments were conducted on dogs and cats, whereas most of today’s experiments are conducted on rats. Considering these methodological limitations, further data are needed to elucidate the role of muscle mass on the pressor response to contraction. In addition, no data are available concerning the effect of muscle mass on the pressor response to stretch (i.e., mechanical stimulus to group III and IV afferents) or lactic acid injections (i.e., metabolic stimulus to group III and IV afferents). These experiments might provide valuable information as to whether the brainstem and the spinal cord process the sensory information arising from mechanoreceptors differently from that arising from metaboreceptors.

The primary purpose of this study, therefore, was to investigate the effect of the amount of muscle mass activated on the exercise pressor reflex in a preparation that had no central command. Using the decerebrate rat model, which was first developed by Smith et al. (22), we tested the hypothesis that the active skeletal muscle mass is an important determinant of the magnitude of the exercise pressor reflex. The secondary purpose of our study was to investigate the effect of the amount of muscle mass in evoking the pressor response to stretch and to lactic acid injection.

METHODS

The surgical procedures and experimental protocols described in this investigation were approved by the Institutional Animal Care and Use Committee of the Penn State College of Medicine. Adult male Sprague-Dawley rats (400–650 g) were housed in temperature-controlled facilities (24°C ± 1°C) and were fed standard chow and tap water ad libitum on a 12:12-h light-dark cycle. The core body temperatures of the rats were maintained at ~37°C–38°C with a heating pad throughout surgical procedures. At the end of the experiment, decerebrated rats were euthanized with an intravenous injection of saturated potassium chloride, and the chests were opened.

Surgical Procedures

The surgical procedures were similar to those performed in a previous study (6). Briefly, rats were first anesthetized with isoflurane in oxygen (3%–4%), the trachea was cannulated, and the lungs were mechanically ventilated (Harvard Apparatus). Immediately after tracheal intubation, maintenance levels of anesthetic gases (2%, oxygen balanced) were given for the remainder of the surgical period. After completion of decerebration, which renders the rat insensate, the lungs were mechanically ventilated with room air. The right and left carotid arteries were cannulated retrogradely with stiff polyurethane tubing (RPT-40 tubing; Braintree Scientific) to measure arterial blood pressures (P23 XL; Statham) and to collect blood for periodic analysis of arterial gases and pH (ABL 80 FLEX, radiometer). Blood gas parameters and pH were maintained within normal limits (35–45 mmHg $\text{P}{\text{a}}_{\text{C}\text{O}}{}_{{}_2}$, >90 mmHg $\text{P}{\text{a}}_{\text{O}}{}_{{}_2}$, pH 7.35–7.45) by either ventilation adjustment or intravenous injection with sodium bicarbonate (8.5%). Intravenous supplementation with starch-saline (0.9%) (50:50) was used maintain blood pressure when necessary.

We decerebrated the rat by sectioning the brain stem 1 mm anterior to the superior colliculi; all neural tissue rostral to the section was removed by suction. Kwik Seal was applied over and at the base of the colliculi to protect the brain from drying and irritation, as well as to occlude bleeding vessels. The cranial cavity was packed with gauze and cotton, and the preparation was allowed to stabilize for 60 min before beginning experimental protocols. Ketamine was given intravenously (0.5 μg) when spontaneous motor behavior was observed.

For protocols 1 and 2 (contraction and passive stretch), the triceps surae muscles and the sciatic nerves were exposed by blunt dissection. The calcaneal tendon was severed and linked with braided fishing line to a force transducer (FT10; Grass Instruments), which in turn was attached to a rack and pinion. The peroneal nerve was cut, and the tibial nerve was carefully isolated by blunt dissection, after which a stimulating electrode was placed under it. Before decerebration, rats were secured within a customized Kopf stereotaxic and spinal frame. Tissues were kept moist using gauze soaked with saline.

For protocol 3 (hindlimb injections), a small (size VO 1.5N) vascular occluder (DocXS, Okiah, CA) was used to trap the injectate within the hindlimb circulation of both legs. Briefly, the femoral vein and adjacent lymphatic vessel were isolated by blunt dissection, and then the occluder was secured around these vessels with 2-0 silk suture. The occluder was inflated with air using a 10-mL syringe fitted with a 21-gauge blunt needle to verify the volume needed for complete occlusion of the occluder lumen. The superficial epigastric arteries (SEA) were then isolated by blunt dissection and cannulated, using 6-0 suture to secure the saline-filled tubing. The injectate was delivered into the arterial supply of the hindlimbs via the polytetrafluoroethylene tubing (SUBL 140 tubing; Braintree Scientific) connected to PinPorts (25 ga, InsTech). To match volume and time between the injections, Hamilton syringes (1.0 mL, GasTight) fitted with PinPort injectors were mounted onto a dual syringe pump (GenieTouch; Kent Scientific).

Experimental Procedures

Protocol 1: bilateral hindlimb contraction versus single hindlimb contraction (n = 8).

To evoke the exercise pressor reflex, the triceps surae muscles were contracted for 30 s by electrically stimulating (40 Hz; 0.01-ms pulse duration) the tibial nerve with constant currents (Grass, Model PSIU6)) that were less than two times motor threshold. Baseline tension was set manually between 50 and 160 g, and contractions were performed following a 30-s baseline period. To verify that the changes in blood pressure during contractions were not the result of electrically activating afferent nerve fibers, pancuronium bromide (0.5 mg/kg iv) was administered to paralyze the rats, and the currents used to contract the muscles were again applied for 30 s. Stimulation of the tibial nerve in pancuronium-paralyzed rats with the currents used to contract the triceps surae muscles did not evoke pressor responses in any of the rats used to collect data.

Protocol 2: bilateral hindlimb stretch versus single hindlimb stretch (n = 8).

To simulate the mechanical component of the exercise pressor reflex, we passively stretched the triceps surae muscles for 30 s by turning the rack and pinion attached to the calcaneal tendon. We compared the pressor responses to stretch of one hindlimb with the pressor responses to simultaneous stretch of both hindlimbs. Baseline tension was set manually between 50 and 160 g, and the stretch maneuver was performed following a 30-s baseline period. The peak tension reached during stretch for each hindlimb was approximately 500 g. A 5-min period elapsed between stretches of the same hindlimb. Throughout the experiment, rats were paralyzed with rocuronium bromide (2 mg/kg iv bolus, followed by 4 mg·kg⁻¹·h⁻¹ iv infusion).

Protocol 3: bilateral hindlimb lactic acid injection versus single hindlimb lactic acid injection (n = 7).

To simulate the metabolic component of the exercise pressor reflex, we injected lactic acid solutions (12 mM, 120 μL) into the SEA catheter at a rate of 2.05 mL/min; we used a dual syringe pump to simultaneously inject the lactic acid solutions into the SEA of both hindlimbs. The dead space in the SEA catheters was 20 μL. Femoral venous outflow was blocked with the vascular occluder 30 s before, during, and 30 s after injections. The catheters were flushed with saline afterwards. Changes in arterial blood pressure relative to baseline values were calculated for a period of 30 s following start of the injection. To verify that changes in blood pressure were not the result of either occluding vessels or injecting a volume of fluid into the circulation, we performed bilateral occlusions with and without saline injections (120 μL, 2.05 mL/min), which was a vehicle for lactic acid, in 3 rats. With the vascular occluder blocking venous flow, we injected blue dye into the SEA to verify both the trapping of the injectate within the hindlimb and the tissue area exposed to the injectate. The rat was immediately euthanized following administration of the blue dye. The lungs and heart were then surgically exposed to verify that the dye did not escape from the hindlimb circulation and stain these tissues. Throughout the experiment, rats were paralyzed with rocuronium bromide (2 mg/kg iv bolus followed by 4 mg·kg⁻¹·h⁻¹ iv infusion). One rat served in protocols 2 and 3. Otherwise, each rat tested served in only one of the three protocols.

Data and Statistical Analysis

All data and measurements were captured with Spike2 (v 7.2) data acquisition system. The data were analyzed using Sigma Plot for Windows v. 11.0 (Systat Software, San Jose, CA). Statistics for peak changes in blood pressure and heart rate evoked by hindlimb static contraction, passive stretch, and lactic acid injections were performed on the difference between 30 s of prestimulus baseline values against 30 s of calculated values at the onset of applied stimulus. Peak values were used for statistical comparison in contraction, passive stretch, and lactic acid experiments. The integral under the curve was used to calculate the blood pressure index (BPI) as well as the tension-time index (TTI). Lactic acid experiments did not include analysis of BPI because of the very short duration of stimulation. Data from right and left hindlimb treatments within each experimental protocol were summed to obtain values for comparison to the bilateral hindlimb treatment data, and the average blood pressure and heart rates for the summed values were used to represent the baseline measurements. The data are expressed as means ± SD and were analyzed by repeated measures ANOVA. When a significant difference was detected, a Holm-Sidak post hoc analysis was applied to detect differences between individual means. The criterion for statistical significance was set at P < 0.05.

RESULTS

Pressor Response to Bilateral and Unilateral Contraction

Figure 1 illustrates the reflexive increase in blood pressure evoked by hindlimb contractions. The peak pressor response to unilateral contraction of the left hindlimb muscles was equivalent to the peak pressor response to unilateral contraction of the right hindlimb muscles (left vs. right, P = 0.842; Fig. 2). Bilateral contractions induced a greater peak pressor response than did contracting either hindlimb alone (bilateral vs. right, P = 0.019; bilateral vs. left, P = 0.023). The peak pressor response to bilateral contraction was not significantly different from the peak pressor response of the arithmetic sum of the response to unilateral contraction (P = 0.143). Peak changes in tension during contractions did not differ between unilateral right and left hindlimb contractions (P = 0.516) or between bilateral contractions and the arithmetic sum of right and left hindlimb contractions (P = 0.345). The significant changes in peak change in tension detected by statistical analysis for all other comparisons were equivalent (P < 0.001) and are illustrated in Fig. 2.

Fig. 1.

Representative blood pressure (BP) waveforms and tension tracings for contraction protocol. A: pressor response evoked by bilateral triceps surae muscles contraction. B: pressor response evoked by right triceps surae muscles contraction. C: pressor response evoked by left triceps surae muscles contraction. Contractions begin at the 20 s marker for all images. The horizontal bar denotes the time when triceps surae muscles were contracted. Time is represented in seconds on the x-axis for all images. Left triceps surae muscles contraction (L) appears adjacent to the corresponding trace; right triceps surae muscles contraction (R) also appears adjacent to the corresponding trace.

Fig. 2.

Blood pressure (A and B) and mechanical (C and D) response to bilateral and unilateral contraction (n = 8 rats). The data are represented by vertical bars as the mean change in peak blood pressure (ΔBP) (A), blood pressure index (BPI) (B), peak change in tension (ΔT) (C), and tension-time index (TTI) (D) to right or left unilateral and bilateral contraction. The arithmetic sum of the left and right unilateral leg is also presented. Note that for every variable the change is induced by unilateral contraction doubles when triceps surae muscles are simultaneously contracted. Baseline blood pressure before each contraction is given below the vertical bars in A. Data were analyzed by one-way repeated measures ANOVA. The horizontal bars represent significant differences between groups (P < 0.05). There was no difference in baseline blood pressures. The groups for comparison are denoted under the horizontal tick marks. Data from individual experiments are represented by closed circles connected by dotted lines. Bilateral, bilateral contraction; Left, unilateral contraction of the left triceps surae muscles; Right, unilateral contraction of the right triceps surae muscles; R+L, sum of right and left unilateral triceps surae muscles contractions.

Analysis of the BPI revealed findings that paralleled those of the peak pressor responses to contraction. Specifically, the magnitude of the BPI response to bilateral contractions was significantly greater than the unilateral contraction-induced BPI response to left (bilateral vs. left, P = 0.007) and right (bilateral vs. right, P < 0.001) hindlimb contraction (Fig. 2). The bilateral contraction-evoked BPI response was not different than that of the arithmetic sum unilateral contraction-evoked BPI response [right (R) + left (L) vs. bilateral, P = 0.105]. Combined peak tension for bilateral contractions was approximately double the tension for unilateral contractions (bilateral vs. right, P < 0.001; bilateral vs. left, P < 0.001). Furthermore, the TTI for contractions was not different between unilateral contractions (left vs. right, P = 0.827) or between the sum of the unilateral and the bilateral contraction TTI (R + L vs. bilateral, P = 0.255). The total TTI for bilateral contractions was approximately double that for unilateral contractions (bilateral vs. left, P < 0.001; bilateral vs. right, P < 0.001). There were no differences detected in baseline blood pressures throughout the protocol (Fig. 2).

Pressor Response to Bilateral and Unilateral Stretch

Figure 3 illustrates the reflexive increase in blood pressure evoked by passively stretching the hindlimbs. The peak pressor responses to stretch of the left hindlimb muscles were not different from the peak pressor responses to stretch of the right hindlimb muscles (P = 0.733). In addition, the peak pressor response to unilateral stretch of either hindlimb muscles was not significantly different from the peak pressor response to simultaneous bilateral stretch of the hindlimbs (bilateral vs. left hindlimb, P = 0.127; bilateral vs. right hindlimb, P = 0.083). Furthermore, the peak pressor response evoked by simultaneous bilateral stretch was not different from the sum of the unilateral peak pressor response evoked from each hindlimb (R + L vs. bilateral, P = 0.399).

Fig. 3.

Representative blood pressure (BP) waveforms and tension tracings for stretch protocol. A: pressor response evoked by bilateral triceps surae muscles stretch. B: pressor response evoked by right triceps surae muscles stretch. C: pressor response evoked by left triceps surae muscles stretch. The passive stretch maneuvers begin at the 20 s marker for all images. The horizontal bar denotes the time when triceps surae muscles were stretched. Time is represented in seconds on the x-axis for all images. L, left triceps surae muscles stretch; R, right triceps surae muscles stretch.

Nevertheless, the BPI evoked by bilateral stretch was approximately double that of either unilateral stretch alone (left vs. bilateral, P = 0.037; right vs. bilateral, P = 0.021) and equivalent to the sum of the unilateral stretch response (R + L vs. bilateral, P = 0.789). There were no differences detected in baseline blood pressures throughout the protocol. These results are summarized in Fig. 4.

Fig. 4.

Blood pressure (A and B) and mechanical (C and D) response to bilateral and unilateral passive stretch (n = 8 rats). The data are represented by vertical bars as the mean change in peak blood pressure (ΔBP) (A), blood pressure index (BPI) (B), peak change in tension (ΔT) (C), and tension-time index (TTI) (D) to right or left unilateral and bilateral passive stretch. The arithmetic sum of the left and right unilateral leg is also presented. Note that the change in peak blood pressure induced by passive stretch is not statistically different than the unilateral response. The blood pressure index, peak tension, and TTI evoked by bilateral stretch were at least two times greater in magnitude compared with the unilateral condition. Baseline blood pressure before each stretch maneuver is given below the vertical bars in A. Data were analyzed by one-way repeated measures ANOVA. The horizontal bars represent significant differences between groups (P < 0.05). There was no difference in baseline blood pressures. The groups for comparison are denoted under the horizontal tick marks. Data from individual experiments are represented by closed circles connected by dotted lines. Bilateral, bilateral stretch of the triceps surae muscles; Left, unilateral stretch of the triceps surae muscles; Right, unilateral stretch of the right triceps surae muscles; R+L, arithmetic sum of the right and left unilateral stretch.

Pressor Response to Bilateral Versus Unilateral Lactic Acid Injections

A bolus injection of 12 mM lactic acid into the superficial epigastric artery evoked a transient pressor response, which we quantified as a peak increase from baseline. Simultaneous bilateral lactic acid injection evoked a pressor response approximately double that of either unilateral lactic acid-evoked responses (left vs. bilateral, P = 0.01; right vs. bilateral, P = 0.017). The unilateral lactic acid-induced pressor responses were similar to each other (P = 0.703). The lactic acid-evoked pressor responses to bilateral injections were approximately equal to those calculated for the unilateral sum (R + L vs. bilateral, P = 0.179). There were no differences detected in baseline blood pressures throughout the protocol. These results are summarized in Fig. 5.

Fig. 5.

Blood pressure (BP) response to bilateral and unilateral lactic acid (LA; 12 mM) injection (n = 7 rats). A: the data are represented by vertical bars as the mean change in peak blood pressure to unilateral and bilateral injections of lactic acid. The arithmetic sum of the left and right unilateral leg is also presented. Note that the peak increase in blood pressure evoked by bilateral lactic acid injection into the hindlimb circulation is twice as great as left or right unilateral injection alone. Baseline blood pressure before each injection is given below the vertical bars. Data were analyzed by one-way repeated measures ANOVA. The horizontal bars represent significant differences between groups (P < 0.05). There was no difference in baseline blood pressures. The groups for comparison are denoted under the horizontal tick marks. Data from individual experiments are represented by closed circles connected by dotted lines. B: representative tracings of the mean arterial pressure following injection of lactic acid into the hindlimb circulation. The arrow indicates the time of lactic acid injection into the hindlimb circulation. Black trace represents LA injection into left hindlimb circulation, light gray trace represents LA injection into right hindlimb circulation, and dark gray trace represents bilateral LA injection into right and left hindlimb circulation. Bilateral, bilateral injection; Left, unilateral injection into the left hindlimb circulation; Right, unilateral injection into the right hindlimb circulation; R+L, arithmetic sum of right and left unilateral hindlimb injections.

Cardioaccelerator Response to Bilateral and Unilateral Stimulation.

Baseline heart rates did not differ at any point during each protocol. Furthermore, contraction, stretch, and lactic acid injections did not produce changes in heart rate that were statistically significant (Table 1).

Table 1.

Heart rate responses to unilateral or bilateral contraction, passive stretch, or lactic acid injection

Protocols	n	Variable	P Value	Bilateral, beats/min	Right, beats/min	Left, beats/min	R + L, beats/min
Contraction	8	Baseline ± SD	0.601	260 ± 64	264 ± 62	265 ± 63	265 ± 62
		∆ Peak ± SD	0.234	4 ± 4	2 ± 4	2 ± 2	4 ± 6
Passive stretch	8	Baseline ± SD	0.059	319 ± 42	298 ± 61	297 ± 50	297 ± 55
		∆ Peak ± SD	0.350	3 ± 11	2 ± 3	8 ± 12	10 ± 11
Lactic acid (12 mM)	7	Baseline ± SD	0.867	323 ± 17	324 ± 52	322 ± 51	323 ± 51
		∆ Peak ± SD	0.740	1 ± 3	1 ± 2	1 ± 2	1 ± 3

Values are means ± SD; n = number of rats. P value has been calculated using one-way repeated measures ANOVA. “Bilateral” denotes bilateral stimulation. “Right” denotes unilateral stimulation of the right triceps surae muscles/hindlimb. “Left” denotes unilateral stimulation of the left triceps surae muscles/hindlimb. R + L denotes arithmetic sum of unilateral triceps surae muscles/hindlimb.

DISCUSSION

The major finding of our study was that the magnitude of the exercise pressor reflex (evoked by static contraction), the muscle mechanoreflex (evoked by tendon stretch), and the muscle metaboreflex (evoked by lactic acid injection) were dependent on muscle mass. Specifically, the greater the muscle mass stimulated by any of the three maneuvers, the greater the magnitude of the pressor response. A second finding was that the effect of muscle mass on the three reflexes examined in our experiments summed additively. We found no evidence of potentiation. A third finding was that the pressor reflex evoked from the right hindlimb was equivalent to that of the pressor reflex evoked from the left hindlimb, provided that both hindlimbs were exposed to stimuli of equal magnitude. This was the case regardless of whether the stimulus evoking the pressor reflex was static contraction, tendon stretch, or lactic acid injection.

Our findings in decerebrated, unanesthetized rats confirmed and extended previous findings in anesthetized dogs and cats (15) as well as previous findings in decerebrated cats (11). The report by McCloskey and Streatfeild (15), in particular, was qualitative in nature and offered no statistical treatment of the data. Specifically, McCloskey and Streatfeild reported that the pressor responses to simultaneous bilateral contraction were not greater than those to unilateral contraction in every cat or dog tested. Interpretation of this statement is difficult. In contrast, we subjected our data to statistical analysis and found that pressor responses to bilateral contraction were significantly greater than those to unilateral contraction. Iwamoto and Botterman (11), using cats, compared the pressor responses to contraction of the lateral and medial gastrocnemius muscles before and after using nerve crush to remove afferent input from the lateral gastrocnemius. This is a different experiment from the one in which we compared the pressor responses to contracting simultaneously the triceps surae muscles of both hindlimbs with the pressor responses to contracting the triceps surae muscles of one hindlimb. Finally, it is important to recognize that current investigations into the exercise pressor reflex exclusively involve rats (9, 13, 19), and information concerning the effect of muscle mass on this reflex is lacking in this preparation. To the best of our knowledge, ours is the first report to provide this information.

The direct effect of muscle mass on the magnitude of the reflex arising from the hindlimb muscles in our experiments seems best explained by the number of thin fiber afferents stimulated by stretch, lactic acid injection, or contraction. We speculate that the dorsal horn of the spinal cord transmitted these thin fiber afferent inputs to brainstem circuitry in a linearly graded fashion, which in turn produced an efferent sympathetic output that manifested as a pressor response. The cardioaccelerator effects evoked by the three maneuvers used in our experiments were small and most likely had little effect on cardiac output. Consequently, we speculate that the pressor responses to the three maneuvers were caused by a sympathetically induced vasoconstriction and not by an increase in cardiac output. Nevertheless, it is important to recognize that, in the decerebrate rat preparation, static contraction of the hindlimb muscles often yields only small cardioacceleration, an effect that is probably due to the loss of resting vagal tone to the heart.

The role played by muscle mass in evoking the cardiovascular responses to static exercise in humans is controversial. Specifically, several investigators have claimed that these responses are independent of the mass of muscle being contracted but are dependent on the percentage of maximum voluntary contraction (7, 14, 15, 24). In other words, the pressor response to exercise should be the same when performing static knee extension as it is when performing static finger exercise. In contrast, other investigators have shown that the pressor response to static exercise was clearly dependent on the mass of contracting muscle (8, 17, 20, 21). In these human experiments, central command is likely to play a role in generating at least some of the cardiovascular responses to static exercise. If that is the case, then the contribution of the exercise pressor reflex to the cardiovascular responses to static exercise in humans described is unclear. To overcome this difficulty, investigators have used postexercise circulatory occlusion as a means to exclude central command from contributing to the cardiovascular responses to static exercise (1). Experiments in which postexercise circulatory occlusion was used to exclude central command revealed that the magnitude of the muscle metaboreflex was dependent on the amount of muscle mass previously contracted statically (10, 15, 17). Nevertheless, it is important to recognize that the “metaboreflex” evoked by postcontraction circulatory occlusion is not the same as the metaboreflex evoked during contraction in which metabolic byproducts interact with a mechanical stimulus on the group III and IV muscle afferents, evoking the exercise pressor reflex (12).

In our experiments, we compared the magnitude of the pressor responses to lactic acid injection and tendon stretch applied to one hindlimb with the magnitude of the pressor responses to these two stimuli when applied simultaneously to both hindlimbs. Our findings were consistent with an interpretation that stimulating group III and IV afferents from a large muscle mass (i.e., both hindlimbs) evoked a larger pressor reflex response than did stimulating these afferents from a small muscle mass (i.e., one hindlimb). One limitation in interpreting our findings with lactic acid is that our method of injection exposed thin fiber skin, muscle, and joint afferents to this stimulus. Consequently, the pressor reflex to lactic acid injection did not arise solely from the stimulation of group III and IV muscle afferents.

The reflex pressor response to passive stretch of the triceps surae muscles was first described in the decerebrated cats (23) and later in decerebrated rats (22). To our knowledge, this is the first report of the reflex pressor response to bilateral passive stretch of the triceps surae muscles in animals. As recently discussed by Copp et al. (4), the pressor reflex evoked by passive stretch in the decerebrated rat is mediated, at least in part, by stretch-sensitive receptors, i.e., Piezo channels on the endings of group III mechanoreceptors. It is interesting to note that the magnitude of the mechanoreceptor-mediated pressor reflex evoked by passive stretch was greater than the reflex pressor reflex evoked by muscle contraction. This observation could be explained by the mechanoreceptor activation differences during passive muscle stretch versus active muscle shortening with concomitant increases in intramuscular pressure (23).

Perspectives and Significance

The exercise pressor reflex evoked by static muscle contraction is the integrated result of mixed metabolic and mechanical signals. Our findings demonstrate for the first time in rats that the amount of muscle mass activated contributes directly to the magnitude of the pressor reflexes arising from the hindlimb muscles. Our findings, nevertheless, do not exclude the possibility that within a given muscle, the discharge rate or number of group III and IV afferents activated by a given stimulus is directly related to the magnitude of the pressor reflex. This possibility suggests that tension development by a given muscle while it is contracting can also impact the magnitude of the exercise pressor reflex. Finally, the rat is the species of choice for the study of the exercise pressor reflex in animals, and our findings provide a basis for future investigations into the role played by muscle mass in evoking this reflex in both physiological and pathophysiological situations.

GRANTS

This study was funded by National Institutes of Health Grants R01-AR-059397 and P01-HL-134609.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.A.E. conceived and designed research; J.A.E. performed experiments; J.A.E. analyzed data; J.A.E. interpreted results of experiments; J.A.E. prepared figures; J.A.E. drafted manuscript; J.A.E., G.P.D., and M.P.K. edited and revised manuscript; J.A.E., G.P.D., and M.P.K. approved final version of manuscript.